Each time I've done long distance (Canada to Mx) rides, I've thought about this "sail" idea. I'm not an engineer, but my concept is to use the wind only when its coming from the rearward direction. The mechanism I have in mind would resemble half of an umbrella when open. Some configuration of strong light fabric attached to lightweight "stays". It would require an easily accessible lever(?) located on the top post or seat post to instantly open to take advantage of the wind, or close the sail, so as not to fight a headwind. I'm not sold on a sail attached to the front set of the bike, and rising like on a sail boat. My gut sez visibility issues, sideways tipping forces, etc While these issues may not be a problem for three wheelers on ice or sand with plenty of maneuvering room, traveling on the side of the road is hazardous enough without adding to it... (but then again, I'm not an engineer) Just a thought.